The invention relates to a method for diagnosing the functioning of a level control system of a motor vehicle, comprising at least one valve, an exchange of pressure medium from the one side of the valve to the other side of the valve being possible when the valve is open and an exchange of pressure medium being prevented when the valve is closed, and comprising a control unit.
Known from DE10126458C2 is a method for testing sealing and functioning wherein, in a first step, a non-return valve being open, there is set at the individual pneumatic spring a stroke at which at least one damping element is compressed, and wherein, in a second step, after the closing of the non-return valve, the individual pneumatic spring is set to a partial stroke of its total stroke between the damping elements, and wherein, in a third step, after the opening of the non-return valve, the ensuing distance between axle and vehicle body is ascertained with the aid of the measuring device and compared with a reference value. A disadvantage of this method is that it is necessary for relatively time-consuming level control operations to be performed for functional testing.
A method of the type stated at the beginning is known from the printed publication DE19959012C2. The method, known from this printed publication, for open-loop and/or closed-loop control of the level of a vehicle body of a motor vehicle is performed by means of a level control system comprising actuators, by means of which the level of the vehicle body can be lowered by discharging an actuating medium from the actuators. In the case of the known method for identifying a malfunctioning of the level control system, the level of the vehicle body is ascertained in relation to a reference point, a discharge operation is initiated if the level that has been ascertained is above a predefined level, it being checked in the control unit, after initiation of the discharge operation, whether after a time period the vehicle body has become lower in relation to the level ascertained immediately before the discharge operation, and the control unit terminating the discharge operation if this is not the case within the time period. The time period is in the range of a plurality of seconds and above.
A disadvantage of the known method is that it takes a relatively long time until a defective functioning of the level control system is identified, and the vehicle may possibly incur damage during this time period. It is also a disadvantage that a malfunctioning of the level control system can be identified only upon discharging. Further, the known method does not permit any inference concerning the cause of the defective functioning, and terminates the level control operation immediately, and possibly does not permit any further level control operation, since the cause is unknown. A discharge operation terminated according to the known method can have a plurality of causes, e.g. lifting by means of a vehicle jack, resting on an object, a blocked discharge line or also, alternatively, a defective control line or a defective valve.
Object of the invention is to create an improved method for the rapid identification of a defective functioning of a level control system, whereby the cause of the fault can at least be localized.